Pressure sensitive adhesives that are appropriate for use in removable tape applications require a fourfold balance of peel, tack, adhesion, and resistance to low stress peel. Especially important is the balance between adhesion and resistance to low stress peel.
Masking tapes provide one example of a removable tape. Masking tapes are often used in the automotive industry to mask surfaces during painting. Typically, a masking tape is applied to a surface, exposed to elevated temperature and/or a chemical environment that often includes organic solvent, and removed when the tape user has finished the task. The tape must be easy to apply, stay in place without lifting or curling under conditions of high temperature and chemical environment, and remove cleanly and easily without breaking, damaging the surface, or leaving adhesive residue.
Pressure sensitive adhesives based on non-thermoplastic hydrocarbon elastomers such as natural rubber may be readily formulated to provide an adhesive that meets the requirements of a masking tape. The dominant means of processing such adhesives involves dissolving the elastomer and other adhesive components in a hydrocarbon solvent, coating the solution onto a backing, and drying the coated product to remove the solvent. These solvent-based processes have become increasingly undesirable, however, because of the environmental and safety considerations associated with the use of solvents.
Environmental and safety considerations have led to accelerated interest in the use of hot melt extrusion coating of adhesive compositions. The elastomers typically employed in this technique are "thermoplastic" elastomers of the block copolymer type, including, for example, styrene-isoprene block copolymers.
Although adhesives based on thermoplastic elastomers eliminate the need for solvent-based processing, their adhesive properties are different than those of adhesives based on non-thermoplastic elastomers, and they are not suitable for some applications. For example, conventional star monoalkenylarene/conjugated diene block copolymers (such as star styrene-isoprene-styrene (SIS) block copolymers) are generally not suitable for the formulation of removable adhesives. When formulated to give adhesion in the range desirable for a removable tape, the adhesive lacks the strength to prevent lifting, and the peel and unwind are often not smooth. When formulated to have sufficient strength to prevent lifting, the adhesion is too high, and the tape is difficult to remove.
Patent literature discloses a wide variety of block copolymer structures which are useful in the formulation of adhesives. For example, St. Clair (U.S. Pat. No. 4,391,949) discloses and claims a block copolymer having the structure (A-B).sub.x Y(C).sub.z wherein A is a poly(monoalkenyl) block, B and C are poly(conjugated diene) blocks, Y is a residue of multifunctional coupling agent, and x plus z is greater than 6. St. Clair (U.S. Pat. No. 4,444,953) discloses and claims an adhesive composition comprising the block copolymer described in U.S. Pat. No. 4,391,949. Hansen (U.S. Pat. No. 4,133,731) discloses and claims that the block copolymer in a pressure sensitive adhesive can be chemically crosslinked by including a multifunctional acrylate or methacrylate crosslinking agent in the pressure sensitive adhesive formulation and exposing the adhesive to high energy radiation such as electron beam or ultraviolet radiation. Erickson (U.S. Pat. No. 5,104,921 ) discloses a cured adhesive composition prepared by high energy ionizing radiation initiated curing of a polymer composition comprising an alkenylarene/conjugated diene block copolymer and an oligomer such that the unsaturation index of the composition is minimized. The radiation initiated curing of the adhesive is accomplished without requiring the aid of a coupling agent to promote crosslinking of the block copolymer during exposure to the radiation. Heinz (U.S. Pat. No. 4,430,417) discloses a photopolymerizable mixture of one or more block copolymers which are solely elastomeric and comprise two or more elastomeric polymer blocks having a glass transition temperature (Tg) of from -20.degree. C. to 15.degree. C., linked by one or more elastomeric polymer blocks having a glass transition temperature of below -20.degree. C. Lau (U.S. Pat. No. 4,780,367) discloses a pressure sensitive adhesive composition comprising a star block copolymer having the general structure (A-B).sub.n C wherein A is a terminal polymeric block consisting essentially of polymerized monovinyl aromatic monomer having 8 to 18 carbon atoms and selected from the group consisting of styrene and alkylated styrene; B represents a polymeric block consisting essentially of polymerized conjugated diene monomer having 4 to 12 carbon atoms; C represents the residue of a polyvinyl aromatic compound providing a nucleus which links together the arms of the star block copolymer; and the number of arms, represented by n, is at least 12.
St. Clair (U.S. Pat. No. 4,556,464) discloses pressure-sensitive adhesives made with ABA star block copolymers wherein the endblock (1) is a random copolymer of a monoalkenylarene and a conjugated diene and (2) has a Tg between 19.degree. C. and 100.degree. C. The copolymer endblock includes a diene in order to introduce a reactive site into the endblock for crosslinking. St. Clair shows that tack loss can be improved by adding a crosslinking additive that is mainly compatible with the endblock phase to preferentially promote crosslinking in the endblock over the midblock.
The present invention provides a novel multi-arm block copolymer, particularly useful in the formulation of a pressure sensitive adhesive. The adhesive of the invention can be hot melt extrusion coated to provide a variety of different types of tapes, especially removable tapes having sufficient resistance to low stress peel while maintaining moderate peel adhesion. Such tapes may be used as masking tapes, packaging tapes, medical tapes and autoclave indicator tapes. Additionally, the pressure sensitive adhesive may be used to make adhesive-backed protective sheeting, labels, and facestock.